Fractional distillation column control

ABSTRACT

A desired cloud point temperature for a side draw product stream from a fractional distillation column is maintained at a desired level by comparing a calculated actual cloud point temperature, which is updated periodically by a cloud point measurement, to a desired cloud point temperature with results of the comparison being utilized to control the flow rate of the side draw product stream so as to maintain the actual cloud point temperature of the side draw product stream substantially equal to the desired cloud point temperature. Use of the calculated cloud point temperature provides a means by which continuous control of the actual cloud point temperature of the side draw product stream can be maintained.

This invention relates to control of a fractional distillation column.In one aspect this invention relates to method and apparatus forcontinuously controlling the cloud point temperature for a productstream drawn from the side of a fractional distillation column ("sidedraw product").

As used herein the term "end point temperature" refers to a temperatureat which all of a liquid has boiled off. The term "cut pointtemperature" refers to a temperature at which some percentage of aliquid has boiled off. The term "cloud point temperature" refers to atemperature at which a fractional distillation product becomes cloudy oropaque which generally indicates that wax or other substances arebeginning to crystallize or separate from the fractional distillationproduct. The term"internal reflux to vapor ratio" refers preferably to amass flow rate ratio but a volume flow rate ratio may be utilized ifdesired since either of these ratios may be derived from a knowledge ofthe other ratio.

The cold flow properties of products from a crude oil distillation towerare important especially when these products are used in geographicallocalities having cold ambient temperatures. This is especially true ofthe heavier products such as the heavy gas oil. One cold flow propertytypically specified is the cloud point temperature.

In the past, control of a cloud point temperature for a particularproduct stream has generally been implemented by utilizing an analysisof the product stream to determine the actual cloud point of thatparticular product stream. Control of the cloud point temperature isthen based on such analysis. However, the interval between tower changesand cloud point measurements is generally on the order of about an hourand it is very difficult to maintain continuous control of cloud pointbased on such intermittent measurements.

It is thus an object of this invention to provide method and apparatusfor continuously controlling the cloud point temperature for a side drawproduct stream withdrawn from a fractional distillation column.

In accordance with the present invention, method and apparatus isprovided whereby a calculated actual cloud point temperature, which isupdated periodically by a cloud point measurement, is compared to adesired cloud point temperature with the results of the comparison beingutilized to control the flow rate of the side draw product stream so asto maintain the actual cloud point of the side draw stream substantiallyequal to the desired cloud point. In general, the actual cloud point maybe determined by determining the actual internal reflux to vapor ratiobased on a basic material balance and heat balance around the specificarea of the column which is of concern for the particular side drawproduct stream. The thus determined actual internal reflux to vaporratio is then utilized to determine the actual end point or cut point ofthe particular sidedraw product stream and this end point or cut pointis utilized to determine the actual cloud point of the sidedraw productstream. The thus calculated cloud point is corrected periodically bycomparing the calculated cloud point to the periodically provided actualcloud point measurement to derive a correction factor which is appliedto the calculated cloud point during the period between actual cloudpoint measurements. In this manner, an actual cloud point is availablecontinuously which enables continuous control of the actual cloud pointof the sidedraw product stream.

Other objects and advantages of the invention will be apparent from theforegoing brief description of the invention and the claims as well asthe detailed description of the drawings in which:

FIG. 1 is an illustration of a fractional distillation column and theassociated control system for maintaining a desired cloud pointtemperature for a side draw product stream withdrawn from the fractionaldistillation column;

FIG. 2 is a diagrammatic illustration of the various flows of vapor andliquid at the top of the fractional distillation column illustrated inFIG. 1;

FIG. 3 is a diagrammatic illustration of the various flows of liquid andvapor in the central portion of the fractional distillation columnillustrated in FIG. 1;

FIG. 4 is a logic diagram for the computer logic utilized to generatethe control signals utilized in the control of the fractionaldistillation column illustrated in FIG. 1;

FIG. 5 is a typical plot of the internal reflux to vapor ratio as afunction of temperature for the heavy gas oil side draw stream;

FIG. 6 is a typical plot of the internal reflux to vapor ratio as afunction of temperature for the naphtha side draw stream;

FIG. 7 is a typical plot of the internal reflux to vapor ratio as afunction of temperature for the kerosene side draw stream; and

FIG. 8 is a typical plot of cloud point temperature as a function of the50% cut point for a particular crude oil feedstock.

The invention is illustrated and described in terms of a crude tower forfractionating a crude oil feed. However, the invention is alsoapplicable to other types of fractional distillation columns in which itis desired to control the cloud point temperature of a product streamflowing from the fractional distillation column. Also, the invention isdescribed in terms of the control of the cloud point of one productstream but is applicable to the control of the cloud point of combinedproduct streams.

A specific control system configuration is set forth in FIG. 1 for thesake of illustration. However, the invention extends to different typesof control system configurations which accomplish the purpose of theinvention. Lines designated as signal lines in the drawings areelectrical or pneumatic in this preferred embodiment. Generally, thesignals provided from any transducer are electrical in form. However,the signals provided from flow sensors will generally be pneumatic inform. Transducing of these signals is not illustrated for the sake ofsimplicity because it is well known in the art that if a flow ismeasured in pneumatic form it must be transduced to electrical form ifit is to be transmitted in electrical form by a flow transducer. Also,transducing of the signals from analog form to digital form or fromdigital form to analog form is not illustrated because such transducingis also well known in the art.

The invention is also applicable to mechanical, hydraulic or othersignal means for transmitting information. In almost all control systemssome combination of electrical, pneumatic, mechanical or hydraulicsignals will be used. However, use of any other type of signaltransmission, compatible with the process and equipment in use, iswithin the scope of the invention.

A digital computer is used in the preferred embodiment of this inventionto calculate the required control signal based on measured processparameters as well as set points supplied to the computer. Analogcomputers or other types of computing devices could also be used in theinvention. The digital computer is preferably an OPTROL 7000 ProcessComputer System from Applied Automation, Inc., Bartlesville, Oklahoma.

Signal lines are also utilized to represent the results of calculationscarried out in a digital computer and the term "signal" is utilized torefer to such results. Thus, the term signal is used not only to referto electrical currents or pneumatic pressures but is also used to referto binary representations of a calculated or measured value.

The controllers shown may utilize the various modes of control such asproportional, proportional-integral, proportional-derivative, orproportional-integral-derivative. In this preferred embodiment,proportional-integral-derivative controllers are utilized but anycontroller capable of accepting two input signals and producing a scaledoutput signal, representative of a comparison of the two input signals,is within the scope of the invention.

The scaling of an output signal by a controller is well known in controlsystem art. Essentially, the output of a controller may be scaled torepresent any desired factor or variable. An example of this is where adesired flow rate and an actual flow rate is compared by a controller.The output could be a signal representative of a desired change in theflow rate of some gas necessary to make the desired and actual flowsequal. On the other hand, the same output signal could be scaled torepresent a percentage or could be scaled to represent a temperaturechange required to make the desired and actual flows equal. If thecontroller output can range from 0 to 10 volts, which is typical, thenthe output signal could be scaled so that an output signal having avoltage level of 5.0 volts corresponds to 50 percent, some specifiedflow rate, or some specified temperature.

The various transducing means used to measure parameters whichcharacterize the process and the various signals generated thereby maytake a variety of forms or formats. For example, the control elements ofthe system can be implemented using electrical analog, digitalelectronic, pneumatic, hydraulic, mechanical or other similar types ofequipment or combinations of one or more such equipment types. While thepresently preferred embodiment of the invention preferably utilizes acombination of pneumatic final control elements in conjunction withelectrical analog signal handling and translation apparatus, theapparatus and method of the invention can be implemented using a varietyof specific equipment available to and understood by those skilled inthe process control art. Likewise, the format of the various signals canbe modified substantially in order to accommodate signal formatrequirements of the particular installation, safety factors, thephysical characteristics of the measuring or control instruments andother similar factors. For example, a raw flow measurement signalproduced by a differential pressure orifice flow meter would ordinarilyexhibit a generally proportional relationship to the square of theactual flow rate. Other measuring instruments might produce a signalwhich is proportional to the measured parameter, and still othertransducing means may produce a signal which bears a more complicated,but known, relationship to the measured parameter. Regardless of thesignal format or the exact relationship of the signal to the parameterwhich it represents, each signal representative of a measured processparameter or representative of a desired process value will bear arelationship to the measured parameter or desired value which permitsdesignation of a specific measured or desired value by a specific signalvalue. A signal which is representative of a process measurement ordesired process value is therefore one from which the informationregarding the measured or desired value can be readily retrievedregardless of the exact mathematical relationship between the signalunits and the measured or desired process units.

Referring now to the drawings, and in particular to FIG. 1, there isillustrated a fractional distillation column 11 which is utilized tofractionate a crude oil feed into a variety of products. For the sake ofsimplicity, only the overhead product, bottoms product and one side drawproduct are illustrated in FIG. 1. The crude oil feed is supplied to thefractional distillation column 11 through the combination of conduitmeans 12 and furnace 13. The crude oil feed is heated to a desiredtemperature in the furnace 13 prior to entering the fractionaldistillation column 11.

An overhead stream is provided from the fractional distillation column11 through conduit means 15 to the heat exchanger 16. The heat exchanger16 is provided with a cooling medium flowing through conduit means 7.The fluid stream from the heat exchanger 16 is provided to the overheadaccumulator 18 through conduit means 21. Liquid in the accumulator iswithdrawn from the accumulator through conduit means 23. The fluidflowing through conduit means 23 is provided as an upper external refluxto the fractional distillation column 11 through the combination ofconduit means 23 and conduit means 24. The fluid flowing through conduitmeans 23 is also provided as the overhead product from the fractionaldistillation column 11 by the combination of conduit means 23 and 25.

A side draw stream is withdrawn from the fractional distillation column11 through conduit means 28. The side draw stream flowing throughconduit means 28 may be considered representative of any side drawstream that may be withdrawn from a crude tower such as the heavynaphtha side draw stream, light gas oil side draw stream, kerosene sidedraw stream or heavy gas oil side draw stream. Since all side drawstreams are treated in the same manner for control purposes, only oneside draw stream is illustrated for the sake of clarity.

The side draw stream flowing through conduit means 28 is provided to theseparator 29. A heating fluid flows to the separator 29 through conduitmeans 31. The separator 29 is utilized to separate some of the lightercomponents from the side draw stream 28. The separated lightercomponents are returned to the fractional distillation column 11 throughconduit means 33. A side draw product stream is withdrawn from theseparator 29 through conduit means 34.

A bottoms stream generally containing reduced crude oil is withdrawnfrom the fractional distillation column 11 through conduit means 36. Anumber of other process streams would generally be flowing to or beingwithdrawn from the fractional distillation column. However, for the sakeof simplicity, these remaining process streams have not been illustratedsince they play no part in the invention and also, the many pumps,additional heat exchangers, additional control components and othertypical fractional distillation column equipment have not beenillustrated.

Temperature transducer 41 in combination with a temperature measuringdevice such as a thermocouple, which is operably located in conduitmeans 15, provides an output signal 42 which is representative of thetemperature of the overhead stream flowing through conduit means 15.Signal 42 is provided from the temperature transducer 41 as an input tocomputer means 100.

Temperature transducer 44 in combination with a temperature measuringdevice such as a thermocouple, which is operably located in conduitmeans 24, provides an output signal 46 which is representative of thetemperature of the upper external reflux flowing through conduit means24. Signal 46 is provided from the temperature transducer 44 as an inputto computer means 100.

Flow transducer 51 in combination with the flow sensor 52, which isoperably located in conduit means 25, provides an output signal 54 whichis representative of the flow rate of the overhead product streamflowing through conduit means 25. Signal 54 is provided from the flowtransducer 51 as an input to computer means 100.

Flow transducer 56 in combination with the flow sensor 57, which isoperably located in conduit means 24, provides an output signal 58 whichis representative of the flow rate of the upper external reflux flowingthrough conduit means 24. Signal 58 is provided from the flow transducer56 as an input to computer means 100.

Temperature transducer 63 in combination with a temperature measuringdevice such as a thermocouple, which is operably located in conduitmeans 28, provides an output signal 65 which is representative of thetemperature of the side draw stream flowing through conduit means 28.Signal 65 is provided from the temperature transducer 63 as an input tocomputer means 100.

Flow transducer 67 in combination with a flow sensor 68, which isoperably located in conduit means 34, provides an output signal 69 whichis representative of the flow rate of the side draw product streamflowing through conduit means 34. Signal 69 is provided from the flowtransducer 67 as an input to computer means 100 and as an input to theflow controller 71.

Flow transducer 91 in combination with the flow sensor 92, which isoperably located in conduit means 12, provides an output signal 93 whichis representative of the flow rate of the crude oil feed flowing throughconduit means 12. Signal 93 is provided from the flow transducer 91 asan input to computer means 100.

Cloud point analyzer 95 is in fluid communication with conduit means 34through conduit means 96. The cloud point analyzer may be a Cloud PointMonitor from Hone Instruments, Ltd., London, England. The cloud pointanalyzer 95 provides an output signal 97 which is representative of theactual cloud point of the side draw product stream flowing throughconduit means 34. Signal 97 is provided from the cloud point analyzer 95as an input to computer means 100. Typically, a period of about an hourwill elapse between the time a sample is taken by the cloud pointanalyzer 95 and the time an analysis of the sample is available.

In response to the described input signals, computer means 100calculates the flow rate of the side draw product stream flowing throughconduit means 34 required to maintain a desired cloud point temperaturefor the side draw product stream flowing through conduit means 34.Signal 81, which is representative of the flow rate of the side drawproduct stream flowing through conduit means 34 required to maintain adesired cloud point temperature for the side draw product stream flowingthrough conduit means 34, is provided from computer means 100 as the setpoint input to the flow controller 71. The flow controller 71 providesan output signal 83 which is responsive to the difference betweensignals 69 and 81. Signal 83 is provided to the control valve 85 whichis operably located in conduit means 34. The control valve 85 ismanipulated in response to signal 83 to thereby maintain the actualcloud point temperature substantially equal to the desired cloud pointtemperature.

FIGS. 2 and 3 will be utilized to illustrate the manner in which theactual internal reflux to vapor ratio in the intermediate portion of thefractional distillation column 11 is calculated. All flow ratesdiscussed are mass flow rates. Conversion of a measured volumetric flowrate to a mass flow rate is well known. In general, the measured volumeflow rate of a fluid is multiplied by the density of the fluid to givethe mass flow rate of the fluid. The density of the various fluidsassociated with any particular fractional distillation process willgenerally be well known but may be calculated from measured data ifdesired. Since this particular conversion is well known, the actualconversion is not described for each fluid stream hereinafter.

Referring now to FIG. 2, the dashed line is preferably the top tray ofthe fractional distillation column 11. A vapor illustrated as V₁ flowsup the fractional distillation column. A portion of the vapor flowing upthe fractional distillation column 11 is cooled when contacted with theupper external reflux flowing through conduit means 24 and is condensed.The portion of the vapor stream flowing up the fractional distillationcolumn 11 that condenses is illustrated as L₁. The upper external refluxflowing down the fractional distillation column is illustrated as L₂.The combination of L₁ and L₂ is equal to the internal reflux flow rateimmediately below the top tray of the fractional distillation column 11.The portion of the vapor flowing up the fractional distillation columnwhich is not condensed is illustrated as V₀. This vapor flows throughconduit means 15. A material balance for the upper portion of thefractional distillation column 11 illustrated in FIG. 1 gives

    (1) V.sub.1 -V.sub.o =L.sub.1 +L.sub.2 -F.sub.x.           (1)

Rearranging Equation (1) and setting L₁ +L₂ equal to R₁, where R₁ isrepresentative of the internal reflux flow rate in the upper portion ofthe fractional distillation column 11, gives

    (2) V.sub.1 =V.sub.o +R.sub.1 -F.sub.x.                    (2)

A heat balance for the upper portion of the fractional distillationcolumn 11 illustrated in FIG. 2 gives

    (3) R.sub.1 F.sub.x (1+KΔT)                          (3)

where ΔT is the difference between the temperature of the vapor flowingthrough conduit means 15 and the temperature of the external refluxflowing through conduit means 24 and K is equal to the specific heat ofthe external reflux divided by the heat of vaporization for the externalreflux. Using the nomenclature of FIG. 1, ΔT is equal to the differencebetween the temperature represented by signal 42 and a temperaturerepresented by signal 46. F_(x) is equal to the flow rate represented bysignal 58 converted to a mass flow rate.

Calculation of the specific heat and heat of vaporization of a fluidsuch as the external reflux flowing through conduit means 24 is wellknown in the art. Reference materials such as The Chemical Engineer'sHandbook, 4th and 5th edition, McGraw-Hill, provide calculations andtables for the specific heat and heat of vaporization of the componentswhich would make up the external reflux flowing through conduit means24.

Since F_(x), K and ΔT are known in Equation (3), Equation (3) can besolved to derive the actual internal reflux in the upper portion of thefractional distillation column 11. R₁ can then be substituted intoEquation (2) and since V_(o) can be determined from the combination ofthe flow rate of the upper external reflux flowing through conduit means24 and the overhead product flowing through conduit means 25, Equation(2) can be solved for the actual mass flow rate of the vapor at the toptray of the fractional distillation column 11.

Referring now to FIG. 3, the boundaries for the material and heatbalance are chosen so as to be able to ignore the effect of the refluxreturning through conduit means 33 except to the extent that refluxaffects V₂ or R₂ illustrated in FIG. 3. V₁ and R₁ are as previouslydescribed with reference to FIG. 2. V₂ is the vapor flow rate at a pointimmediately below the point from which the side draw stream flowingthrough conduit means 28 is withdrawn and in like manner R₂ is theliquid flow rate at that same point. A mass balance for FIG. 3 gives

    (4) V.sub.2 =F.sub.D +V.sub.1 +R.sub.2 -R.sub.1            (4)

An energy balance for FIG. 3 gives ##EQU1## where ΔT is equal to thedifference between the temperature of the liquid above the point wherethe side draw stream flowing through conduit means 28 is withdrawn andthe temperature of the side draw stream flowing through conduit means28. The temperature above the point where the side draw stream flowingthrough conduit means 28 is withdrawn is approximately equal to thetemperature of the overhead stream flowing through conduit means 15.Thus, utilizing the nomenclature of FIG. 1, ΔT of Equation (5) isrepresentative of the difference between the temperature represented bysignal 42 and the temperature represented by signal 65. In Equation (5)h₁ is representative of the heat of vaporization of the side draw streamflowing through conduit means 28 and C_(p) is representative of thespecific heat of the side draw stream flowing through conduit means 28.

All of the elements of Equation (5) are known except for R₂ and thusEquation (5) can be solved for the actual flow rate of the internalreflux at the point immediately below the point where the side drawstream flowing through conduit means 28 is withdrawn from the fractionaldistillation column 11. R₂ can then be substituted into Equation (4) andsince V₁ is known from Equation (2) and R₁ is known from Equation (3),Equation (4) may be solved for V₂. The ratio of the results of Equations(4) and (5) gives the internal reflux to vapor ratio at the pointimmediately below the point where the side draw stream flowing throughconduit means 28 is withdrawn from the fractional distillation column11.

FIGS. 5-7 will be utilized to illustrate the manner in which actual 50%cut point temperature can be determined. Referring to FIG. 5, thestraight line labeled CUT POINT is derived by actual measurements of the50% cut point for a heavy gas oil for a known internal reflux to vaporratio. Once the cut point line is established, the actual internalreflux to vapor ratio, calculated by taking the ratio of Equations 4 and5, may be used to derive the actual 50% cut point of the heavy gas oil,as illustrated. FIGS. 6 and 7 would be utilized for naphtha and kerosenerespectively.

FIG. 8 will be utilized to illustrate the manner in which the actualcloud point can be determined. Referring to FIG. 8, the cloud pointcurve is derived by actual measurements of the cloud point of productsfrom a fractional distillation column which have particular 50% cutpoints. Once the cloud point curve is established, the 50% cut pointdetermined from FIGS. 5-7 may be used to derive the actual cloud pointof the particular product.

It is noted that FIG. 8 is valid for a particular crude oil feed. If thecomposition of the crude oil feed changes substantially, then the curveillustrated in FIG. 8 may also change. However, in general thecomposition of the crude oil feed to a particular fractionaldistillation column will not change substantially for long periods oftime and thus a particular plot of cloud point versus cut point can beutilized for these long periods of time in which the crude oilcomposition does not change substantially.

While determination of the actual cloud point has been described interms of using the actual 50% cut point temperature it is noted thatother cut point temperatures could be utilized or the end pointtemperature could be utilized if a plot of a particular cut pointtemperature or the end point temperature versus cloud point isavailable.

A simplified flow diagram for the computer logic utilized to calculatethe set point signal 81 is illustrated in FIG. 4. Conversion of volumeflow rates to mass flow rates is not illustrated but would be utilizedfor each measured flow rate. Referring to FIG. 4, signal 42, which isrepresentative of the temperature of the overhead stream flowing throughconduit means 15, is provided as an input to block 111 and is alsoprovided as an input to block 117. Signal 46, which is representative ofthe temperature of the external reflux flowing through conduit means 24is also provided to block 111. Signal 58, which is representative of theflow rate of the external reflux flowing through conduit means 24 isprovided as an input to block 111 and is also provided as an input toblock 112. In response to the described input signals, the flow rate(R₁) of the internal reflux in the upper portion of the fractionaldistillation column 11 is calculated utilizing Equation (3). Signal 125,which is representative of the flow rate R₁, is provided from block 111as an input to blocks 112, 117 and 118. Block 112 is also provided withsignal 54 which is representative of the flow rate of the overheadproduct stream flowing through conduit means 25. In response to thedescribed inputs, the flow rate (V₁) of the vapor at the pointimmediately below where the upper external reflux flowing throughconduit means 24 is returned to the fractional distillation column 11 iscalculated utilizing Equation (2). Signal 127, which is representativeof V₁, is provided from the block 112 as an input to block 118.

Signal 65, which is representative of the temperature of the side drawstream flowing through conduit means 28, is provided as an input toblock 117. Signal 69, which is representative of the flow rate of theside draw product stream flowing through conduit means 34, is providedas an input to block 117 and is also provided as an input to block 118.In response to the described inputs, the flow rate (R₂) of the internalreflux at the point immediately below the point where the side drawstream flowing through conduit means 28 is withdrawn from the fractionaldistillation column 11 is calculated. Signal 131, which isrepresentative of R₂, is provided from block 117 as an input to block118 and is also provided to the numerator input of the dividing block119.

In response to the described inputs, the flow rate (V₂) of the vapor atthe point immediately below the point where the side draw steam flowingthrough conduit means 28 is withdrawn from the fractional distillationcolumn 11 is calculated in block 118. Signal 133, which isrepresentative of V₂, is provided from block 118 to the denominatorinput of the dividing block 119. Signal 131 is divided by signal 133 toestablish signal 135 which is representative of the actual internalreflux to vapor ratio at the point immediately below the point where theside draw stream flowing through conduit means 28 is withdrawn from thefractional distillation column 11. Signal 135 is provided from thedividing block 119 as an input to 141.

The plots illustrated in FIGS. 5-8 are entered into the computer and areutilized to calculate the actual cloud point based on the actualinternal reflux to vapor ratio represented by signal 135. The thuscalculated cloud point, which is represented as signal 142, is providedas an input to the delay block 143 and is also provided as an input tothe summing block 144. The delay block 143 is utilized to compensate forthe time period which passes between the time a sample is taken by thecloud point analyzer and the time the results of the cloud pointanalyses are available. This time may typically be on the order of onehour and thus signal 142 would be delayed for one hour by the delayblock 143 to establish signal 146. Essentially, signal 146 would berepresentative of the actual cloud point calculated one hour earlierassuming that the delay between taking of a sample and availability ofan analysis is one hour. Signal 146 is provided from the delay block 143to the subtrahend input of the subtracting block 148.

Signal 97, which is representative of the actual cloud point as measuredby the cloud point analyzer 95 is provided to the minuend input of thesumming block 148. Signal 146 is substrated from signal 97 to establishsignal 149. Signal 149 is provided as an input to the summing block 144.Signal 149 is summed with signal 142 to establish signal 151.

Essentially, signal 149 is utilized to make any correction required inthe calculated actual cloud point. It is noted that, if signal 146 andsignal 97 are equal, than the magnitude of signal 149 will be zero. Onlywhen the calculated actual cloud point and the measured actual cloudpoint differ will a correction factor be applied by the use of signal149.

Signal 151 is provided from the summing block 144 as the processvariable input to the controller block 153. The controller block 153 isalso provided with a set point signal 154 which is representative of thedesired cloud point of the product stream flowing through conduit means34. In response to signals 151 and 154, the controller block 153provides an output signal 156 which is responsive to the differencebetween signals 151 and 154. Signal 156 is scaled so as to berepresentative of the percentage the crude oil feed stream flowingthrough conduit means 12 which must be removed as product throughconduit means 34 in order to maintain a desired cloud point for theproduct flowing through conduit means 34. Signal 156 is provided fromthe controller block 153 as an input to the multiplying block 158.

Signal 93, which is representative of the flow rate of the crude oilfeed flowing through conduit means 12 is provided as an input to thedelay block 159. The delay block 159 is utilized to compensate for thetime required for a change in the flow rate of the crude oil flowingthrough conduit means 12 to cause a change in the flow rate of theproduct flowing through conduit means 34. Signal 93 is delayed in thedelay block 159 to establish signal 161 which is provided as a secondinput to the multiplying block 158. Signal 156 is multiplied by signal161 to establish signal 81 which is representative of the flow rate ofthe product stream flowing through conduit means 34 required to maintaina desired cloud point for that product stream. Signal 81 is utilized ashas been previously described.

It is noted that use of the crude oil feed flow rate provides a means bywhich changes in the crude oil feed flow rate may be compensated for.Without this compensation, substantial time may pass during whichoff-specification product is being made because the flow rate of thefeed has changed.

The invention has been described in terms of a preferred embodiment asillustrated in FIGS. 1-7. Specific components used in the practice ofthe invention as illustrated in FIG. 1 such as flow sensors 52, 57, 68and 92; flow transducers 51, 56, 67 and 91; flow controllers 61 and 71;temperature transducers 41, 44 and 63; and pneumatic control valves 76and 85 are each well known, commercially available control componentssuch as are described at length in Perry's Chemical Engineer's Handbook,4th Edition, Chapter 22, McGRaw-Hill.

While the invention has been described in terms of the presentlypreferred embodiment, reasonable variations and modifications arepossible by those skilled in the art within the scope of the describedinvention and the appended claims.

That which is claimed is:
 1. Apparatus comprising:a fractionaldistillation column; means for providing a feedstream to said fractionaldistillation column; means for withdrawing a side draw stream from anintermediate portion of said fractional distillation column, wherein atleast a portion of said side stream is utilized to provide a side drawproduct stream; computor means for enabling establishment of a firstsignal representative of the actual internal reflux to vapor ratio insaid intermediate portion of said fractional distillation column;computor means for enabling establishment of a second signalrepresentative of the calculated actual cloud point of said side drawproduct stream in response to said first signal; means for measuring thecloud point of said side draw product stream and for establishing athird signal representative of the measured actual cloud point of saidside draw product stream; computor means to enable utilization of saidthird signal to correct any error in the cloud point represented by saidsecond signal to thereby establish a fourth signal representative of acorrected actual cloud point of said side draw product stream;means forestablishing a fifth signal representative of the desired cloud point ofsaid side draw product stream; means for comparing said fourth signaland said fifth signal and for establishing a sixth signal which isresponsive to the difference between said fourth signal and said fifthsignal; and means for controlling the cloud point of said side drawproduct stream in response to said sixth signal.
 2. Apparatus inaccordance with claim 1 wherein said means for establishing said firstsignal comprises means for calculating the value of said first signalbased on a material balance and energy balance for said intermediateportion of said fractional distillation column.
 3. Apparatus inaccordance with claim 2 wherein said means for establishing said secondsignal comprises means for calculating the value of said second signalbased on a plot of the internal reflux to vapor ratio in saidintermediate portion of said fractional distillation column as afunction of the cut point or end point temperature of said side drawproduct stream and based on a plot of the cloud point of said side drawproduct stream as a function of a cut point or end point of said sidedraw product stream.
 4. Apparatus in accordance with claim 1 whereinsaid means for using said third signal to establish said fourth signalcomprises:means for delaying said second signal for a time period equalto the time period which elapses between changes in conditions for saidfractional distillation column and availability of the measured cloudpoint based on sampling of said side draw product stream to therebyestablish a seventh signal; means for subtracting said seventh signalfrom said third signal to thereby establish an eighth signal; and meansfor adding said second signal and said eighth signal to establish saidfourth signal.
 5. Apparatus in accordance with claim 1 wherein saidsixth signal is scaled so as to be representative of the percentage ofsaid feed stream which must be removed as said side draw product streamin order to maintain the actual cloud point of said side draw productstream substantially equal to the desired cloud point and wherein saidmeans for controlling the cloud point of said side draw product streamin response to said sixth signal comprises:means for establishing aseventh signal representative of the actual flow rate of said feedstream; means for delaying said seventh signal by the time required fora change in the flow rate of said feed stream to cause a change in theflow rate of said side draw product stream to thereby establish aneighth signal; means for multiplying said sixth signal and said eighthsignal to establish a ninth signal which is representative of the flowrate of said side draw product stream required to maintain the actualcloud point of said side draw product stream substantially equal to thedesired cloud point; and means for manipulating the flow rate of saidside draw product stream in response to said ninth signal.